1. Field of the Invention
This invention relates to novel nitrone compounds which are suitable for use as a main ingredient of a contrast enhanced material for enhancing an image contrast of a subject, like a mask for photoetching used for fabrication, for example, of semiconductive integrated circuits and also to contrast enhanced materials which comprises the nitrone compounds.
2. Description of the Prior Art
In lithographic techniques, a resist image which is obtained by exposing a photo-resist to light in a desired pattern and developing the pattern has more vertically shaped walls when the contrast of the exposed image is greater. When the exposure is carried out at a very high resolution, the contrast of the exposed image is lowered with the attendant problem that a clear resist image cannot be obtained.
For increasing the resolution and obtaining a high resolution pattern, there have been proposed methods of forming resist patterns wherein a contrast enhanced layer for enhancing the contrast which has an absorption maximum against light having a wavelength of 300 to 450 nm (Japanese Patent Publication No. 62-40697 and Japanese Laid-Open Patent Application No. 62-234148). The nitrone compounds used as a main ingredient for the contrast enhanced material have the following structures. ##STR2##
However, the above-mentioned nitrone compounds are insoluble in water and are soluble only in organic solvents. Accordingly, the contrast enhanced materials using the nitrone compounds cannot be soluble in water. This requires the step of removing the contrast enhanced layer with use of organic solvents prior to the developing step of a photoresist. Alternatively, if an intervening layer is provided, it is essentially required to force the intervening layer and the contrast enhanced layer to be blown off by means of pure water. The former removing step using the organic solvent cannot be performed in existing apparatus, inviting complication of the fabrication procedure. With the latter procedure wherein pure water is used to blow the layer off along with the intervening layer, there is the problem that the contrast enhanced layer cannot be completely blown off but is partly left, thus producing scums. Accordingly, there is the demand for nitrone compounds which are useful as an ingredient for contrast enhanced materials and which do not invite complication of the steps and are soluble in pure water.